Loud speaker



July '12, 1932. H, ROUND 1,867,177

LOUD SPEAKER Filed Jan. 3, 1950 INVENTOR HENRY JOSEPH ROUND ATTORNEY Patented July 12, 1932 UNITED STATES PATENT? OFFICE HENRY JOSEPH ROUND, F WESTFIELD, LONDON, ENGLAND, ASSIGNOR T0 RADIO CORPORATION OF AMERICA, A CORPORATION OF DELAWARE noun SPEAKER Application filed January 3, 1930, Serial No. 418,173, and in Great Britain January 10, 1929.

ticularly, it relates to loud speakers of. the

so-called cone or large diaphragm type, i. e., loud speakers in which the sound is generated by a diaphragm which is generally conical and sufficiently large to reproduce a considerable volume of sound without the use of a horn. More especially the: invention relates to combinations of such loud speakers in which the diaphragms are driven by moving coil or electromagnetic movements which have similar constants. Experiments with good representative loud speakers of the type referred to have shown that the radiation characteristic i. e., the curve connecting sound radiation (ordinates) with frequency (abscissae) is generally fairly flat from somewhere below 100 cycles per sec- 2o. 0nd to about 600 or 700 cycles, after which there is a'tendency for the efficiency toy increase until the radiation reaches a maximum at about 3000 cycles. At frequencies above that at which the maximum occurs, the radiation falls off. The maximum value of radiation is generally from four to ten times the intensity at 200 cycles, and in many cases the characteristic shows a dip between 1000 and 2000 cycles. Of course, the exact shape of the characteristic will vary from loud speaker to loud speaker, depending upon the angle of the cone diaphragm, the mass and "diameter of the moving coil employed to impulse it, the mass and stiffness of the material of the diaphragm, and other design factors in each individual case, but generally speaking the characteristic will be found to approximate that described above.

According to this invention sound reproduction is effected by means of a pair of loud speakers of the type referred to, and means are provided for reducing the high frequency output from one of said loud speakers. The said means, which may conveniently be 5 constituted by a large condenser shunted across one loud speaker, should be such as to commence appreciably to reduce the sound output at about 800 cycles, the reduction increasing with increase in frequency until at about 3000 cycles per second the output from the shunted loud speaker approximates to zero.

In use, the two loud speakers are operated together and adjusted in any well known way to give the best effect. l

The loud speakers may be driven each by a. separate valve.

The general combined effect obtained is that, the output is about double at frequencies up to about 700 cycles, owing to the addition of the outputs from the two loud speakers, While about 800 cycles, the output becomes more and more than. from one loud speaker alone.

The invention is illustrated in the accom- .5 panying drawing.

Referring to the drawing 1 is a radio receiver or other device whose output is fed to a transformer 2 the secondary of which applies voltage between the grids and cathodes 1 of two thermionic valves 3, 4, as shown. The anode circuit of valve 3 contains the primary of a transformer 5 whose Secondary feeds a moving coil cone loud speaker 6 while the anode circuit of valve 4 similarly contains the primary of a transformer 7 whose secondary feeds a second loud speaker 8. The primary (or if desired the secondary) of transformer 7 is shunted by a condenser 9 of such magnitude as to reduce the output from loud so speaker 8 at frequencies above about 800 cycles. If desired a suitably dimensioned choke connected between the anode of valve 4 and the primary of transformer 7 may be employed in place of or in addition to the con- '86 denser 9.

In place of valves 3, 4, in parallel, push pull connected valves may be employed.

Preferably the cone of loud speaker 6 is stiffened as indicated at 10.

It has been found that such stiffening of the unshunted cone for a considerable distance from the coil and-at least for one third of the length of the conetends to reduce to some extent the radiation at its maximum value (at about 3000 cycles) and to increase the output at higher frequencies. This stiffening may be effected by sticking some stifi'er material, such as Bristol board or aluminium foil, over a cone made of the ms d. pannitomarily employed thin paper, or any of the methods described in prior British Patent No. 248,433 may be employed.

It has been found that each of the loud speakers'gives a different frequency characteristic at different angles of hearing, and although the general characteristic along the diaphragm cone axis is as herein described, there is a tendency for frequencies of 3000 cycles and over to be less strongly produced a other angles. For this reason, it has been found diflicult to make the reproduction from 100 to 800 cycles per second quite so intense as the reproduction at about 3000 cycles per second, when considered axially, but a. total reproduction, which is much more faithful to the original transmission (e. g. in broadcasting) is obtained when the output at lower frequencies is about doubled as herein described.

It is important, in order to secure best reproduction of the lowest frequencies, that low tone resonance be avoided and for this reason the conical diaphragm of the loud speakers are suspended, as described in the British Patent No. 294,285, so that the suspelsionresonance is below 40 cycles per secon Having now particularly described and ascertained the nature of my said invention and in what manner the same is to be performed, I declare that what I claim is 1. Sound reproducing apparatus comprising two loudspeakers of the same type and response characteristics, both of said loudspeakers being adapted to normally reproduce sound at both high and low frequencies, means for reducing the high frequency output of one of said loudspeakers only, and stiffening means applied to the diaphragm of the other loudspeaker to increase its output at the higher frequencies.

2. Sound reproducing apparatus comprising two loudspeakers of the same type and response characteristics, both of said loudspeakers being adapted to normally reproduce sound at both high and low frequencies, electrical means for reducing the high frequency output of one of said loudspeakers and physical means as distinguished from electrical means for increasing the high frequency output of the other loudspeaker.

3. Apparatus for reproducing sound commal responses at frequencies above 800 cycles per second.

4. Apparatus for reproducing sound comprising a pair of similar loudspeakers each of which has a lower response at low frequencies than at high frequencies, means for energizing both loudspeakers over the entire frequency range to which they will respond, and means for reducing the response of only one of said loudspeakers at the higher frequencies whereby the combined response of the loudspeakers at the lower frequencies will be equal to the sum of their normal responses, and at the higher frequencies will be equal to substantially onlv the response from one of the loudspeakers.

HENRY JOSEPH ROUND.

prising two large-diaphragm, moving-coil loudspeakers having similar response characteristics, connections for energizing their moving coils in parallel and means for reducing the electrical input to one of said loudspeakers at frequencies above 800 cycles per second, whereby the combined response of the loudspeakers will be equal to approximately the sum of their normal responses at 

